Field
The present disclosure generally relates to the design of photonic integrated circuits (PICs). More specifically, the present disclosure relates to a PIC that includes a tunable grating coupler.
Related Art
Grating couplers are important silicon-photonic optical components because they can be used as an input/output (I/O) optical device for optical waveguide-to-fiber surface-normal coupling, as well as for inter-layer optical waveguide-to-optical waveguide coupling on the same chip or between two different chips. A grating coupler enables high-density interconnects, two-dimensional I/O from the surface of a silicon chip, multi-wavelength operation, and the ability to perform wafer-scale testing and binning before the reticles or chips are separated from the wafer. However, these many useful and necessary features often cannot be individually optimized. Consequently, there are typically significant tradeoffs that limit their use in broad-spectrum, wavelength-division-multiplexing (WDM) interconnects, including: manufacturing tolerances, wavelength-passband, lateral and angular alignment tolerances, and optical loss. FIG. 1 presents a block diagram illustrating a side view of an existing integrated circuit with a grating coupler on a silicon optical waveguide for surface-normal coupling.
For a given wavelength λ, when the phase-matching condition in Eqn. 1 is met, the optical mode inside the optical waveguide is coupled out of the optical waveguide at a coupling angle θ with respect to the vertical, or vice versa. In particular,
                                          k            ⁢                                                  ⁢                          sin              ⁡                              (                θ                )                                              +                      p            ⁢                                                  ⁢                                          2                ⁢                π                            Λ                                      =        β                            (        1        )            where k=2π/λ is the modulus of the out-coupled wave vector, p is the diffraction order, Λ is the grating period, β=(2π/λ)·neff is the real part of the propagation constant and neff is the mean effective index of refraction along one grating period.
In order to obtain high-diffraction efficiency (i.e., low coupling loss) for a receiving optical device having a fixed receiving angle, the grating period Λ is usually selected so that there is only one diffraction order that satisfies the phase-matching condition for the selected wavelength λ. Such a grating coupler is a wavelength-sensitive optical device. For a given grating period Λ and an effective index of refraction neff, the phase-matching angle θ is different for different wavelengths, resulting in different coupling loss to a fixed receiving optical device. Similarly, because of manufacturing tolerances, neff can vary from grating coupler-to-grating coupler and from wafer to wafer, which in turn causes center-wavelength uncertainty for the grating couplers. Consequently, in order to achieve the targeted center wavelength, and to avoid bandwidth narrowing because of center-wavelength variation in a system that uses pre-selected wavelength band(s), or in applications where grating couplers are used for optical waveguide-to-optical waveguide coupling, very tight tolerance control is usually required on both the silicon-layer thickness in silicon-on-insulator (SOI) wafers (which are used to implement silicon photonic components) and the grating trench etch depth. However, it is very difficult to achieve the desired center-wavelength accuracy, because the silicon layer thickness of an SOI wafer typically varies a few percent across the wafer and from wafer to wafer. In addition, there is often a variation of a few nanometers in the etch depth of grating trenches in a grating coupler. Therefore, for a grating coupler on a sub-micron SOI platform, both of the aforementioned variations can cause the center wavelength to deviate from the desired center wavelength by a few to tens of nanometers.
Hence, what is needed is a grating coupler without the problems described above.